Many wireless communication systems communicate voice and data information between communication infrastructure and user equipment using a standard wireless communication protocol. Data information includes information necessary for network browsing, messaging, and multimedia applications. In order to develop improved wireless communication systems having higher communication speeds than existing systems, new standards for managing and handling these improved systems are desired.
Methods for handling high speed communications include High Speed Downlink Packet Access (HSDPA) service and High Speed Uplink Packet Access service (HSUPA). HSDPA is a method for supporting downlink packet data that has been generally defined. Currently, standardisation efforts include the definition of HSUPA for efficiently supporting packet data communication in the uplink direction.
HSDPA and HSUPA use a number of similar techniques including incremental redundancy and adaptive transmit format adaptation. In particular, HSDPA and HSUPA provide for modulation formats and code rates to be modified in response to dynamic variations in the radio environment. Furthermore, HSDPA and HSUPA use a retransmission scheme known as Hybrid Automatic Repeat reQuest (H-ARQ). In the H-ARQ scheme, incremental redundancy is provided by a use of soft combining of data from the original transmission and any retransmissions of a data packet. Thus, when a receiver receives a retransmission, it combines the received information with information from any previous transmission of the data packet. The retransmissions may comprise retransmissions of the same channel data or different channel data may be transmitted. For example, retransmissions may comprise additional redundant data of a Forward Error Correcting (FEC) scheme. The additional encoding data may be combined with encoded data of previous transmissions and a decoding operation may be applied to the combined data. Hence, the retransmission may effectively result in a lower rate (higher redundancy) encoding of the same information data.
Although HSDPA and HSUPA use many similar techniques, HSUPA provides a number of additional complications with respect to HSDPA and not all techniques used for the downlink transmissions are directly applicable to the uplink scenario. In particular for UMTS, scheduling of data for communication over the air interface is performed by the network rather than in the mobile devices. Specifically for HSDPA and HSUPA, aspects of the scheduling are performed in the individual base stations scheduling a user in order to minimize scheduling delays. This permits the air interface communication to be adapted to the dynamic variations in the radio environment and facilitates link adaptation.
For HSDPA the data to be transmitted is available at the base station and in particular the base station includes downlink transmit data buffers. Furthermore, HSDPA provides for transmissions to be made from only one base station and does not support soft handovers where the same data is simultaneously transmitted from a plurality of base stations to the same mobile device. Accordingly, the scheduling by the base station is relatively simple as the information required is available at the base station and as the scheduling by one base station may be made independently of other base stations.
However, in HSUPA, the data to be scheduled is the data which is to be transmitted from the mobile devices. Accordingly, it is important to have an efficient signaling scheme between the mobile devices and the base stations in order to allow the base stations to schedule data from the mobile devices and for the mobile devices to operate in accordance with the scheduling.
Furthermore, HSUPA provides for the use of soft handovers in which a transmission from a mobile device may be simultaneously received by a plurality of base stations with the received signals being combined in the network. However, as the scheduling is performed by one base station in HSUPA, other base stations do not have any information on when the mobile device may transmit. Accordingly, all base stations which may be involved in a soft handover, continuously attempt to receive data transmissions from the mobile device. This requires that the base stations continuously despread the received signals with all spreading codes of mobile devices which potentially may be active. However, as the mobile devices typically transmit only for a fraction of the time, this results in a very high resource usage and in particular results in a large part of the computational resource of the receiver being used to monitor for potential transmissions from mobile devices. Similarly, mobile devices must monitor the downlink signaling from each base station involved in a soft handover where each base station may use multiple channelization codes to send the necessary signaling to the mobile stations thus requiring the mobile to decode multiple channelization codes to determine when and if it is being signalled. Mobile station complexity increases when it must decode multiple channelization codes per cell and channelization codes from multiple cells hence the number of channelization codes a mobile station must decode should be minimized. Also depending on the application type, some forms of scheduling are more optimal than others. For example, ‘time and rate’ scheduling is better when tight control of the quality of server (QoS) is required or larger overhead from signaling can be tolerated. For example, this might occur in a ‘hot spot’ coverage area where the mobile stations are quite close to the base station and may even have line of site. Other types of scheduling such as ‘rate control’ scheduling may be better when only low signaling overhead can be tolerated such as in multi-coverage regions in a macro-cell topology or the applications tend to be best effort like uploading e-mail or low data rate streaming. For applications with low latency requirements the frame size is also important since this can drive much of the physical delay component of the end to end delay experienced by the user (mobile station). A larger frame size can help with coverage issues typically occurring at the edge of the cell or network. Hence, both small and large frame sizes (i.e. small and large transmission time intervals—TTIs) are useful when supported by a high speed uplink packet access network and mobile stations.
There is a need for a system and method for minimizing the number of channelization codes that user equipment must monitor per active set cell. There is a further need for a downlink signaling structure that supports 'rate control and ‘time and rate’ scheduling as well as small and large frame sizes.